Commercially available robotic grippers fall into two general categories characterized by the motion of the gripping jaws. In one category, the gripping jaws move parallel to one another. In another, the gripping jaws move through an angular displacement.
Parallel motion jaws have two distinct advantages over angular motion jaws in that the gripping force vectors intersect inside of the envelope of the jaws for any part within their gripping range of motion and therefore produce no force vector tending to eject the part from the jaws and second, the action of opening and closing the jaws introduces no component of motion to the gripped part in a direction toward or away from the body of the gripper.
The disadvantages of linear translation, parallel jaw motion grippers are that they have more moving parts, weigh more and have a smaller range of jaw motion than angular motion jaw grippers.
Typically, parallel jaw motion grippers have a total range of jaw motion which is only 5 to 25 percent of the diameter of the gripper body, so unless a very large, massive gripper is used, frequent gripper changes are needed in assembly operations involving multiple, different size parts. The time required to change grippers, the positional inaccuracies in mounting different grippers on the robot arm and the variations in jaw centering from one gripper to another all detract from the performance of the assembly cell. Oversize grippers compromise the load carrying capacity and dynamic response of the robot by their increased weight. They also develop considerable clamping forces which are difficult to repeatably control when handling delicate plastic parts. Control is further compromised by piston seal stiction in the pneumatic actuators for these grippers.
Moreover, current torque limitation of small stepping motors limits the grasping force for a jaw to be between 1 and 2 pounds. This restricts their use to grasping objects weighing less than 20 oz. which includes a great majority of the parts in portable modern electronic consumer products.
Planetary gear grippers sold by Techno Sommer Automatic offer a large range of parallel jaw motion in a small size gripper by actuating the jaws through arcs of circles to simultaneously converge or diverge. These grippers overcome most of the mass and force control limitations of linear motion parallel jaw grippers but the inability to control the position of the jaws other than being fully open or fully closed introduces other limitations.
All of the commercially available grippers are actuated through simple pneumatic actuators which allow only fully open or fully closed jaw positions. With this limitation, large range of motion grippers are disadvantaged in gripping parts which are small relative to their maximum jaw opening in that cycle time is wasted in waiting for the jaws to close a large distance and when acquiring and releasing a part, are likely to interfere with adjacent parts. This will often prevent part acquisition or will damage parts which have already been placed in an assembly. Manually adjustable stops are available for some actuators, but it is more practical to change grippers than to adjust the gripper travel while it is installed on the robot. A second problem with pneumatically actuated, long stroke grippers is that high jaw velocities result when the gripper is optimized for cycle time. This results in high impact forces between the jaws and the part being acquired and can lead to part damage and missed parts when the part rebounds off of the jaw.
Additional flexibility is afforded in being able to acquire parts which are presented on their sides or upside down relative to their desired orientation. The parts are brought into the desired orientation by adding a rotational capability to the gripping surfaces of the parallel jaw gripper where the axis of rotation of the part is parallel to the direction of jaw movement. Techno Sommer Automatic offers two jaw parallel jaw grippers with this feature but, once again, the same problem of manually adjustable but nonprogrammable motion renders these pneumatic devices less than suitable for flexible assembly.
In U.S. Pat. No. 4,598,942 to Shum et al, a gripper device is disclosed that uses a planetary gear driven by a D.C. motor for causing movement of the gripper fingers. A shortcoming of this device is that the movements of the fingers are not programmably displaceable for short stroke motions.
U.S. Pat. No. 5,215,507 to Bonig describes a gripper assembly in which a complex gearing arrangement driven by a hydraulic or electric motor is used to produce movement of the gripper fingers. The gripper assembly of Bonig lacks, among other features, an important programmable motor drive for producing short stoke linear motion.
Therefore, a need persists for flexible assembly apparatus and method having a universal gripper design capable of the following functions: acquiring, reorienting and releasing a wide range of part shapes and sizes; operating in confined spaces; automatically resizing its jaw opening to adapt to different size parts; controlling the gripping force exerted on the parts; and, reorienting overturned parts.